1. Field of the Invention
The present invention relates to a high-hardness metal skin film, and particularly, to a metal skin film formed of an aggregate of metal crystals and having a high hardness.
2. Description of the Prior Art
Conventionally known metal skin films of this type include an Fe-plated layer provided on an outer peripheral surface of a base material of steel, for example, in a piston pin for an internal combustion engine in order to improve the wear resistance.
Under existing circumstances where high speed and high output of the internal combustion engine are desired, however, the known metal skin film has a problem that the oil-maintaining property, namely, the oil retention, is not sufficient and the seizure resistance is poor, due to the slide surface being relatively flat.
Therefore, the present applicant has previously developed a metal skin film having a large number of pyramid-shaped metal crystals in a slide surface (for example, see Japanese Patent Application Laid-open No. 174089/94 and U.S. patent application Ser. No. 08/162,520, filed Dec. 3, 1993, now U.S. Pat. No. 5,445,684).
If the metal skin film is formed in the above manner, the adjacent metal crystals assume mutually biting states and therefore, the slide surface takes on an intricate aspect including a large number of fine crest portions, a large number of fine valley portions formed between the crests, and a large number of fine swamps formed due to the mutual biting of the crests. Therefore, the metal skin film has a good oil retention. This improves the seizure resistance of the metal skin film.
As a result of various examinations of the metal skin film, it has been ascertained that the metal skin film is relatively low and hence, in order to allow the skin film to be adapted to a severe sliding environment, it is necessary to increase the hardness of the metal skin film and to inhibit the wear of the pyramid-shaped metal crystals in the sliding environment to maintain the good oil retention of the metal skin film.
It has been also ascertained that in the severe sliding environment, the tip end portions of the pyramid-shaped metal crystals are plastically deformed so that the valley portions of the slide surface are deformed and eliminated, and the tip end portions are worn into an indented configuration in such a manner that they are torn off. As a result, the oil retention by the valley portions is reduced, but also the friction coefficient is increased and for this reason, the sliding characteristic is substantially reduced.
In order to improve the thermal conductivity of the metal skin to efficiently diffuse the frictional heat generated with the sliding movement, it is convenient that the metal skin is formed to have a large number of columnar metal crystals grown from the base material rather than being formed as an aggregate of granular metal crystals. This is because each of the columnar metal crystals serves as a good medium for transferring heat to the base material.
In the metal skin containing a large number of columnar metal crystals as forming components, however, there is a problem that grain boundaries exist parallel toga direction of growth of the columnar metal crystals and for this reason, when a tensile load is applied in a direction intersecting the direction of growth, the grain boundaries are liable to be cracked. There is also a problem that the columnar metal crystals are liable to be relatively increased in grain size and hence, it is impossible to sufficiently increase the hardness of the metal skin.